Relationship between Joint Stiffness, Limb Stiffness and Whole-Body Center of Mass Mechanical Work across Running Speeds

The lower-extremity system acts like a spring in the running stance phase. Vertical stiffness (K-vert) and leg stiffness (K-leg) reflect the whole-body center of mass (COM) and leg-spring system loading and response in running, while joint stiffness (K-joint) represents joint-level dynamic loading and response. This study aimed to investigate whether K-joint is associated with K-vert and K-leg across different running speeds. Twenty healthy subjects were recruited into a treadmill running study (1.8 to 3.8 m/s, with 0.4 m/s intervals). We found that K-joint accounted for 38.4% of the variance in K-vert (p = 0.046) and 42.4% of the variance in K-leg (p = 0.028) at 1.8 m/s; K(joint )also accounted for 49.8% of the variance in K-vert (p = 0.014) and 79.3% of the variance in K-leg (p < 0.0001) at 2.2 m/s. K-knee had the strongest unique association with K-vert and K-leg at 1.8 and 2.2 m/s. Kjoint was associated with Kleg at a wider range of speeds. These findings built a connection between joint stiffness and limb stiffness within a certain range of running speeds. Kknee may need to be considered as an important factor in future limb stiffness optimization and general running performance enhancement.